In recent years, as an audio amplifier for a vehicle or the like, a class-D amplifier adopting a PWM modulator has been spreading. The PWM modulator performs pulse width modulation of a sound signal by comparing the sound signal with a carrier of a sawtooth wave (one side edge) or a triangular wave (both side edges). The PWM modulator obtains a PWM pulse which is a binary switching waveform as a result of the pulse width modulation. The class-D amplifier amplifies power by driving an output transistor (switching transistor) by the PWM pulse from the PWM modulator. Power-amplified switching output is demodulated using a low-pass filter and a speaker is driven.
Incidentally, maximum output power is one element that determines a performance of an audio amplifier. In thermal design of a device, power loss at the maximum output power needs to be taken into consideration.
However, when a circuit is designed in consideration of the maximum output power, there is a problem that loss of an output transistor increases and there is an adverse effect of heat generation in normal use during which only the power sufficiently smaller than the maximum output power is consumed.